[inductor] Use decomposition for smooth_l1_loss_backward (#100242)
Summary: This forward fixes a CI check failure introduced by
https://github.com/pytorch/pytorch/pull/99429. I also updated
auto-labeler rule to trigger inductor test for any decomposition
changes.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/100242
Approved by: https://github.com/bertmaher
